1. Field of the Invention
The invention relates generally to magnetic field sensors, and more particularly to a magnetic field sensor with a graphene sense layer.
2. Description of the Related Art
A magnetic field sensor based on extraordinary magnetoresistance (EMR) has been proposed as a magnetoresistive read-head sensor for magnetic recording hard disk drives. The sense layer in an EMR sensor formed of nonmagnetic III-IV semiconductor materials does not suffer from the problem of magnetic noise that exists in read-head sensors based on giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR), both of which use magnetic films as their sense layers. The operation and structure of EMR sensors with III-IV semiconductor sense layers for read-head applications are described by S. A. Solin et al., “Nonmagnetic semiconductors as read-head sensors for ultra-high-density magnetic recording”, Appl. Phys. Lett., Vol. 80, No. 21, 27 May 2002, pp. 4012 4014; and in U.S. Pat. No. 7,167,346 B2; U.S. Pat. No. 7,170,722 B2 and U.S. Pat. No. 7,203,036 B2; all assigned to the same assignee as this application.
More recently an EMR sensor with a graphene sense layer has been proposed. A graphene magnetic field sensor provides a graphene sense layer only a few atomic layers thick and is thus promising for the detection of nanometer-sized magnetic domains. Graphene is a single atomic sheet of graphitic carbon atoms that are arranged into a honeycomb lattice. It can be viewed as a giant two-dimensional Fullerene molecule, an unrolled single wall carbon nanotube, or simply a single layer of lamellar graphite crystal. Charge carrier mobility values as high as 200,000 cm2/Vs at room temperature are achievable, as described by Morozov et al., PRL 10, 016602, 2008. Pending application Ser. No. 12/539,437 filed Aug. 11, 2009 and assigned to the same assignee as this application, describes a tunable graphene magnetic field sensor. The magnetic field sensitivity and the electrical resistance of the graphene sensor can be tuned by the electric field effect, with the highest sensitivity being when the electrical resistance is at its peak, namely when the electric field from a gate bias voltage penetrating the graphene sense layer causes charge transport to take place via electrons and holes simultaneously. In this high sensitivity regime, the response of the sensor as a function of the applied external magnetic field exhibits a super-linear dependence, with minimum sensitivity around zero magnetic field.
Thus it is desirable to impose a static magnetic biasing field to the graphene sensor such that the operating regime of the sensor is shifted to sensitivity values otherwise unattainable. The static magnetic biasing field has the additional advantage of linearizing the signal response of the sensor, which is a desirable feature for electrical detection of the sensor's response. However, the static magnetic biasing field must not be so great as to adversely affect the write head or the magnetic recording media if the sensor is used as a magnetoresistive read head in a magnetic recording disk drive. Also, if a ferromagnetic layer is used as the static magnetic biasing field it must be located in close proximity to the graphene sense layer without causing electrical shorting of the graphene sense layer.
What is needed is a graphene magnetic field sensor with a ferromagnetic biasing layer that provides the desired static magnetic biasing field and is located in close proximity to the graphene sense layer without causing electrical shorting of the graphene sense layer.